The present invention relates generally to steam precipitation methods for producing polycarbonate resin powder from polycarbonate solutions in nonaqueous solvent (e.g., methylene chloride). More specifically, the present invention relates to a more energy efficient method of steam precipitation which produces a powder having a relatively lower water content, thereby avoiding the need for a substantial portion of the post dryer capacity that would otherwise be required to produce an acceptably dry polycarbonate powder.
Polycarbonates are a widely used class of thermoplastic materials, which are prized for their superior clarity and physical toughness. Worldwide yearly production of polycarbonates is greater than 1.5 million metric tons.
One preferred method of producing polycarbonates, which is commonly referred to as the xe2x80x9cinterfacial method,xe2x80x9d comprises reacting phosgene and bisphenol-A in a two-phase system having an aqueous and a nonaqueous phase wherein the nonaqueous phase typically comprises methylene chloride as a solvent. Upon reaction, a solution of polycarbonate in methylene chloride is typically formed. Next, the aqueous phase is typically removed and the methylene chloride phase may be washed to help remove residual salts, catalysts and other impurities.
In the final drying step, the polycarbonate in methylene chloride solution must be converted to a dry polycarbonate powder. This step typically accounts for a large fraction of the total production cost because drying the powder is a very energy intensive process, which typically employs hot gas dryers. These dryers require large amounts of electricity and steam to operate and are maintenance intensive. The required electricity and steam are expensive to produce, and their production involves an environmental cost. The drying step is also problematic because the capacity of interfacial polycarbonate synthesis plants is often limited by their capacity to perform this final drying step.
Steam precipitation is a commonly employed industrial method for converting polycarbonate in methylene chloride solution to wet polycarbonate powder. In steam precipitation, the polycarbonate solution is atomized and sprayed into a flowing steam atmosphere. The steam atmosphere is at a sufficient temperature and pressure to cause methylene chloride to evaporate from the small droplets of polycarbonate solution, forming granular particles, which are conventionally recovered in a downstream precipitation piping loop.
Unfortunately, while steam precipitation is relatively efficient at vaporizing the methylene chloride, the steam itself simultaneously partially condenses on the particles, forming a wet powder. The wet powder typically has a water content of from about 25 to about 60 percent by weight. Several downstream dryers are necessary to remove the residual water from the wet powder to produce a dry product powder having an acceptable percent water content (wt.), which is typically less than 1 percent.
By the present invention, Applicants have discovered an improved steam precipitation method for producing polycarbonate powder having a relatively lower water content than powder produced by conventional steam precipitation. The improved method enables the production of dry powder using less energy (i.e., steam and electricity) and potentially fewer dryers (with less maintenance).
A polycarbonate powder having a relatively lower water content may be produced by preheating a solution of polycarbonate in a nonaqueous solvent (e.g., methylene chloride) prior to subjecting said solution to steam precipitation. A dry product powder is then produced by subsequently further drying the relatively dry powder produced via said steam precipitation process. Preferably, the solution is preheated to a temperature above 130xc2x0 F., and the steam to resin weight ratio is maintained between 1.2 and 2.5.
In a more specific embodiment, the process for isolating a polycarbonate comprises heating a solution comprising polycarbonate and methylene chloride to a temperature of above 130xc2x0 F., but below the temperature sufficient to cause decomposition of the polycarbonate. The heated solution is then atomized by spraying the heated solution out of a nozzle into a flowing steam atmosphere. Upon contact with the steam, the nonaqueous solvent evaporates, leaving a wet powder. The steam-to-resin weight ratio used in this process should be between 1.2 and 2.5.